Editing IS-IS (section)

== Packet types ==
IS-IS adjacency can be either broadcast or point-to-point.

; IS-IS Hello PDU (IIH) : The IS-IS hello packets needs to be exchanged periodically between 2 routers to establish adjacency. Based on the negotiation, one of them will be selected as DIS (Designated IS). This hello packet will be sent separately for Level-1 or Level-2. There are 3 IS-IS hello packets depending on the circuit type - 
:* '''LAN L1''' ('''PDU type 15''')
:* '''LAN L2''' ('''PDU type 16''')
:* '''P2P''' ('''PDU type 17''').   As can be seen, on point-to-point links there are no separate hello packets per level, while on broadcast links - there are. In IS-IS, compared to OSPF, hello timers do not need to match.
; Link State PDU (LSP) : This contains the actual route information. This LSP can contain many type–length–values (TLVs). LSP header is called '''LSP ID''' and consists of '''System ID''', '''Pseudonode ID''' and '''Fragment ID'''. In example of LSP ID 1921.6820.0002.02-01
:* '''1921.6820.0002''' is '''System ID''' (that generated this LSP),
:* '''02''' is '''Pseudonode ID,'''
:* '''01''' is '''Fragment ID'''.
:If '''Pseudonode ID''' is equal to zero, then this is a '''real''' intermediate system. Any value different from zero means that this LSP is generated by DIS (Pseudonode).
:If LSP is too big, then it gets fragmented. In order to indicate this, Fragment ID is used. If '''Fragment ID''' is '''equal to zero''', then '''no fragmentation''' has occurred.
; Complete Sequence Number PDU (CSNP) : This packet will be sent only by the DIS. By default, for every 10 seconds, CSNP packet will be transmitted by DIS. This will contain the list of LSP IDs along with sequence number and checksum.
; Partial Sequence Number PDU (PSNP) : If the router which receives CSNP packet finds some discrepancy in its own database, it will send an PSNP request asking the DIS to send specific LSP back to it.